This invention relates to the continuous catalytic nitric acid oxidation of cycloalkanols and/or cycloalkanones to saturated aliphatic dicarboxylic acids, and to apparatus for conducting the process.
The oxidation of cycloalkanols and/or cycloalkanones with nitric acid to produce dicarboxylic acids is well known. See U.S. Pat. Nos. 2,439,513; 2,557,282; 2,831,024; 2,844,626; 2,878,276; 3,673,248; British Pat. No. 567,525; French Pat. No. 1,428,374. This process advantageously is conducted at low operating temperatures, such as about 20.degree.-50.degree. C. The prior-filed application of Rohl et al., Ser. No. 28,564, filed Apr. 15, 1970, claims such a process. The thus-produced dicarboxylic acid, in most cases, separates in the solid phase and tends to cling to the walls of the reactors in the form of an insulating crystalline layer. Theretofore, it has not been possible heretofore to successfully employ tube bundle heat exchange (tube-nest) reactors with external cooling, which are otherwise very effective in strongly exothermic reactions, unless the reaction is conducted in the homogeneous phase by the use of special measures, for example, very large dilution or the use of solubilizers.
German Pat. No. 844,144 mentions in passing the possibility of employing a vertically disposed tube bundle (tube-nest) surrounded by a cooling jacket as a reactor. However, no example is set forth in this connection and the specification does not give any further details.
German Published Application DAS No. 1,238,000, on the other hand, gives reasons why a tube-nest reactor should not be used, especially with respect to the process claimed in German Pat. No. 844,144. The utilization of a plurality of tubes of small cross section for the reaction results in difficulties in control technology, primarily because of the difficulty of feeding the reaction mixture through individual heat exchange tubes uniformly and cooling the contents of the various tubes to the same temperature. When there are different reaction conditions in the individual tubes, varying amounts of reaction gas are formed, so that the differences with respect to residence time and temperature characteristic in the individual tubes are magnified, resulting in a reduction in yield. Furthermore, due to deposition of the dicarboxylic acids on the walls, the narrow tubes of a tube-bundle heat-exchange reactor can easily be clogged, thus impairing the continuous operation of the process.
In U.S. Pat. Nos. 2,439,513 and 2,557,282, a reactor is described for use in a process for the preparation of adipic acid by nitric acid oxidation wherein the circulation vessel consists of a pipe cooled on all sides by a cooling jacket. Circulation in the reactor is provided by a pump. These patents also discuss the difficulty of product crystallizing on the cooled walls. Experiments conducted with a device as described in these United States patents and operated in accordance with the directions given in those patents, but using in place of a simple twin-jacket pipe an externally cooled tube nest, were not successful because the tubes continuously become clogged by the crystalline compound, due to insufficient circulation of the reaction sludge.
It is an object of this invention to eliminate the above-described disadvantages and to employ, in the above-described continuous oxidation with recycle of the reaction sludge and gases, a cooled tube-nest (tube bundle) heat exchanger, without the undesirable deposition of the thus-produced dicarboxylic acids on the tube walls, which makes maintenance of the reaction sludge at a uniform temperature impossible.